Our group reported on ACS Nano: Important progress in silica aerogel fiber
    Silica aerogel is a porous material with unique properties such as low density, high porosity, high specific surface area and low thermal conductivity. It has been widely used in the fields of environment, energy, catalysis, and architecture. The preparation process mainly involves sol-gel, aging and drying processes. Due to its slow gel process, it is difficult for silica aerogels to form continuous fibers, which limits its application. In view of this, the group of Prof. Xuetong Zhang from the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences explored the key factors required to achieve fiber formation and developed a reaction spinning method for the preparation of aerogel fibers. Through the structural design of the precursor and the post-treatment of the fiber, a new type of silica aerogel fiber with adjustable transparency and hydrophilicity was developed, and it was verified that the method can also be applied to titanium oxide aerogel fiber.    
    Firstly, by changing the degree of hydrolysis of TEOS, a spinning solution—the silica cluster (CS, commonly known as silicone oil) solution is prepared, and a gel catalyst with a suitable concentration—ammonia ethanol solution is used as a coagulation bath, which solves the key problem of fast gel. When the gel speed is greatly reduced from the minutes or even days to only 5 to 15 seconds, the CS nanoparticles pass through the rapid gel reaction instead of diffusing in the coagulation bath during the fiber forming process and constructed a three-dimensional network structure, as shown in Figure 1.
Figure 1 (a) Preparation of silica aerogel fiber (b-e) The influence of different hydrolysis degree of pre-spinning polymer on gel time and structure
    This work breaks through the traditional perception that traditional wet spinning is highly dependent on the viscosity of the spinning solution, and opens up a potential way for continuous fiberization based on low-viscosity inorganic nanoparticles. For example, using ethanol-diluted tetrabutyl titanate as a spinning solution and an acetic acid aqueous solution as a coagulation bath, titanium oxide aerogel fibers are also obtained through the reaction spinning and followed supercritical drying, as shown in Figure 2.
Figure 2. Preparation of titanium oxide aerogel fiber by reaction spinning method
    Then, the group analyzed the influence of the degree of aggregation of CS cluster particles on the optical properties of the fiber. The aggregation degree of CS particles in the spinning solution can be precisely controlled by the pH value during the hydrolysis process. When the concentration of HCl used is only 0.01 M, CS particles with a size of 24 nm are obtained, and aerogel fibers with a transparency of up to 92% are obtained by reaction spinning; when the concentration of HCl is 0.1 M, the degree of condensation of CS particles Without significant change, the particles are more likely to aggregate, and the aggregate size is uniformly reduced, and the average size is higher than 50 nm. As a result, the obtained aerogel fiber is opaque, and it is milky white when observed with the naked eye. Figure 3 shows the obtained aerogel fibers with different optical properties. The analysis of light scattering mechanism revealed that the main scattering phenomena of two kinds of fibers are Rayleigh scattering and Mie scattering. It is also because of Rayleigh scattering that transparent fibers show a hint of bluish under natural light.
Figure 3 (a-c) Silica aerogel fibers with different transparency
    The silica aerogel fiber obtained by reaction spinning has multi-functional properties, as shown in Figure 4. Since the covalent connection between the particles formed by the gel reaction is no different from the bulk material, the aerogel fibers exhibit exactly the same mechanical properties as the bulk material, and because of its fibrous shape, it also shows high flexibility that the bulk material does not have, and it is not affected by the drastic change of the ambient temperature. In addition, by the treatment of hydrophobization, the originally super-hydrophilic aerogel fibers are transformed into super-hydrophobic fibers due to the methyl functional groups introduced on the surface, their mechanical and thermal properties are not reduced. Even the hydrophobized fiber will have better thermal insulation performance in liquid nitrogen. 
Figure 4 (a) Mechanical properties of aerogel fibers (b) Flexibility of aerogel fibers at the high and low temperature (c) Optional hydrophilicity and hydrophobicity of aerogel fibers (d) Temperature insulation performance of aerogel fibers
    This work reported a more applicable and popularized reaction spinning method to prepare inorganic aerogel fibers. The spinning mechanism comes from the restriction mechanism formed by the gel speed and particle diffusion. Under fast reaction conditions, the particles form a three-dimensional covalently connected network instead of the effectively diffusing. Through the adjustment of the physical and chemical properties of the reactive particles, and the functionalization treatment after spinning, the resulting aerogel fibers can integrate functional properties that are difficult for many bulk materials. And this fibrous structure, especially the transparent aerogel fiber, undoubtedly has very broad application prospects.
    The above related results were published in the international academic journal ACS Nano. The first author of the article is Yu Du, a 2020 PhD student at the University of Science and Technology of China, and Prof. Xiaohua Zhang from the Textile Technology Innovation Center of Donghua University. The corresponding author is Prof. Xuetong Zhang from the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences. The collaborators include Prof. Yezi You from the University of Science and Technology of China and Dr. Jin Wang from the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences and others from Chinese Academy of Sciences. The above work was supported by the key research and development plan of the Ministry of Science and Technology, the National Natural Science Foundation of China, and the key deployment projects of the Chinese Academy of Sciences.

Link to the article:
Reaction-Spun Transparent Silica Aerogel Fibers, https://pubs.acs.org/doi/10.1021/acsnano.0c05016